Automatic-chrominance-control system



June 13, 1961 D. RICHMAN AUTOMATICCHROMINANCE-CONTROL SYSTEM OriginalFiled Nov. 16, 1956 3 Sheets-Sheet 1 SOUND a BEAM- 0 CIRCUITS cDEFLECTION o clRguITs a +2 [2 3 E? I5 I RECEIVER MONOCHROME- VIDEO J 0INPUT 0 a o D SIGNAL -p CIRCUITS DETECTOR AMPLIFIER g E- I- BAND-PASSAMPLIFIER E L J E s GATED BURST AMPLIFIER t .1360.

PFAs?l DETECTOR 2e 334 QUAISRATUTRE LTRANSFORMER FLYBACK PULSE I FROMBEAM DEFLECTION cIRcu Ts A P c REACTANCE 3.6 MC 0 D BUFFER D 0-) FILTERTUBE oscILLAToR AMPLIFIER A P c LOOP "I 4 I l FIG.1

D. RICHMAN 2,988,592

AUTOMATIC-CHROMINANCECONTROL SYSTEM 16, 1956 3 Sheets-Sheet 2 June 13,1961 Original Filed Nov.

NO 0 m T? n w w T Vi c R 7 E l E 1 m w m m2 P l-l. I |l||| |l| 4 S S w Ae e o 2 W M m m m m illllllli W W m H B B O m vi m U m B D m w m 32:0mlmumm 32:0 W m P G 2 4 e. e F F AMPLIFIER ACC BIAS FIG. 5 FROM DETECTORUnited States Patent 2 988 592 AUTOMATIC-cHRoMiNANcE-coN'rRoL SYSTEMDonald Richman, Fresh Meadows, N.Y., assignor to Hazeltine Research,Inc., Chicago, 11]., a corporation of 1 Claim. (Cl. 1'78-'5.4)

General This invention relates to the color-signal or chrominance-signaldecoder of a color-television receiver and, particularly, toautomatic-chrominance-control (ACC) systems for use in such signaldecoders to stabilize the gain of the chrominance signal.

This application is a divisional application of application Serial No.622,702, filed November 16, 1956, now Patent No. 2,936,332.

Automatic-chrominancecontrol (ACC) in the chrominance channel of a colorreceiver is for the same purpose as automatic-gain-control (AGC) in ablack-and-white television receiver and automatic-volume-control (AVC)in a sound receiver, namely, to hold the signal gain relativelyconstant. In color receivers, it is customary to utilize an AGC systemfor automatically controlling the gain of the radio-frequency andintermediate-frequency amplifiers. This serves to stabilize the gain ofthe monochrome as well as the chrominance portion of the color signal. Aseparate gain-control system, in this case an ACC system, is,nevertheless, needed to further control the gain of the chrominancesignal to compensate for variations in the gain of such chrominancesignal relative to the monochrome signal. Such variations arise becauseof the relatively wide band width of the color signal and because themonochromeand chrominance-signal components are primarily located atopposite ends of the band. As a result, variations in antenna impedancematch with frequency and variations in signal strength, due to multipathtransmission, may produce substantial relative variations between themonochrome and chrominance signals.

It has been heretofore proposed to obtain automatic chrominance controlby having at least one amplifier stage in the chrominance channel passboth the chrominancesignal proper and the subcarrier synchronizing burstand then to separate and rectify the burst to obtain an ACC bias whichis then supplied back to the amplifier stage to control the gainthereof. This constitutes a simple form of automatic-gain-control and,as is known in the art, a better type of control action can be obtainedif, instead, a delayed gain-control system is utilized. Delayedautomatic-gain-control systems heretofore proposed generally utilize abiased diode which is coupled to the source of gain-control bias andwhich acts to keep the gain-control system disabled until the receivedsignal reaches a desired strength. In this manner, the amplifier systembeing controlled may be designed to have optimum gain for weak signals,the automatic-gain-control action being disabled until the signalamplitude reaches a desired value and then going into operation fairlyrapidly to hold the signal amplitude at approximately this desiredvalue. It would be desirable to incorporate such delayed control actioninto the chrominance channel ACC system.

Another necessary feature of the chrominance-signal decoder is acolor-killer circuit for disabling the chrominance channel during thereception of a black-and-white picture signal. Unless such circuit isutilized, the blackand-white picture would suffer from color distortionarising from the fact that the synchronous detectors of the chrominancechannel are not properly synchronized during l such reception.

Applicant has found a way of combining the color-killer and ACC circuitsof the chrominance channel so as to obtain the advantage of delayautomatic-gain-control without added expense or circuit complexity overthat required for the color-killer circuit.

It is an object of the invention, therefore, to provide a new andimproved automatic-chrominance-control system.

It is another object of the invention to provide a new and improvedautomatic-chrominance-control system for providing delayed gain controlfor the chrominance channel with a minimum of added expense and circuitcomplexity.

It is a further object of the invention to provide a new and improvedautomatic-chrominance-control system which combines the color-killer andACC circuitry to obtain delayed ACC operation at no increased expenseover that required for the color-killer circuit.

In accordance with the invention, an automatic-chrominance-controlsystem for use in the chrominance-signal decoder of a color-televisionreceiver comprises a chrominance-signal amplifier stage for translatinga received chrominance signal including the subcarrier synchronizingbursts and a chrominance-signal synchronous demodulator stage fordetecting a pair of color-difference signal components of thechrominance signal. The system also includes detector circuit meanscoupled to the output of the amplifier stage and responsive to thesubcarrier synchronizing bursts for developing a control potentialrepresentative of the amplitude of the bursts. The system furtherincludes circuit means for supplying the control potential to thesynchronous demodulator stage to disable this stage when the burstamplitude is insuiiicient and to develop a voltage-delayed outputpotential when the burst amplitude is sufiicient. In addition, thesystem includes circuit means for supplying the voltage-delayed outputpotential back to the amplifier stage in a degenerative manner forstabilizing the gain of the chrominance signal.

For a better understanding of the present invention, together with otherand further objects thereof, reference is had to the followingdescription taken in connection with the accompanying drawings, and itsscope will be, pointed out in the appended claims.

Referring to the drawings:

FIG. 1 is a circuit diagram, partly schematic, of a representativeembodiment of a color-television receiver including a representativeembodiment of an automaticchrominance-control system constructed inaccordance with the present invention;

FIG. 2 is a graph used in explaining the operation theautomatic-chrominance-control system of FIG. 1; FIG. 3 is a circuitdiagram of a chrominance-signal decoder including another form ofautomatic-chrominance-control system constructed in accordance with thepresent invention;

FIG. 4 is a graphutilized in explaining the operation of theautomatic-chrominance-control system of FIG. 3, and

FIG. 5 is a circuit diagram illustrating an alternative form ofamplifier system for use with the automaticchrominance-control system.

Color receiver of FIG. 1

The monochrome portion of thedet ected color Signal;

is applied by way of a monochrome-signal amplifier 15 to the cathodes ofa color picture tube 16 of, for example, the three-gun shadow-mask type.The fieldand line-synchronizing pulses contained in the detectedcomposite color signal are supplied to beam-deflection circuits'17 forcontrolling the generation of the usual scanning'wave forms forproducing the scanning raster of the picture tube 16.

The chrominance-signal portion of the composite detected signal, as wellas the subcarrier synchronizing burst, are supplied by way of aband-pass amplifier 20 to a further amplifier 21. The chrominance signalis then applied to a pair of synchronous detectors 22 and 23. Locallygenerated reference signals of subcarrier frequency, namely 3.6megacycles, and which are in phase quadrature are also supplied to thesynchronous detectors 22'and 23 and serve to control the operationthereof so that a pair of color-difference signals is individuallyderived from the chrominance signal. These color-difference signals arethen supplied to a matrix 24 for producing the red, green, and bluecolor-difference signals which are, in turn, supplied to the individualcontrol electrodes of the color picture tube 16. Such picture tube isthen effective to combine these color-difference signals with themonochrome signal applied to the cathodes to produce the desired colorimage on the phosphor screen of the tube 16.

The locally generated reference signals supplied to the synchronousdetectors 22 and 23 are obtained from a controlled oscillator circuitwhich is maintained in synchronism with the subcarrier synchronizingburst by means of an automatic-phase-control (APC) loop or system. Moreparticularly, the subcarrier synchronizing burst portion of thechrominance signal is obtained from the band-pass amplifier 20 andseparated from the chrominance-signal proper by a gated burst amplifier26 and then supplied to an automatic-phase-control (APC) phase detector28. The burst amplifier 26 is gated by flyback pulses which may beobtained from the beamdeflection circuits 17 and supplied thereto by wayof the terminal 26a. Coupled in cascade to the output of the APC phasedetector 28 are an APC filter 29, a reactance tube 30, a 3.6-megacycleoscillator 31, a buffer amplifier 32, and a quadrature transformer 33.These latter units 2833 make up the automatic-phase-control loop.

Considering briefly the operation of the APC loop, oscillations from thefree-running 3.6-megacycle oscillator 31 are supplied by way of thebuffer amplifier 32 back to the phase detector 28. The phase detector 28comprises, basically, a pair of diode detector circuits coupled in phaseopposition. One detector circuit includes a diode 36, a condenser 37, aresistor 38, and an upper half of the secondary winding of transformer39. The other detector circuit includes a diode 40, a condenser 41, aresistor 42, and a lower half of the secondary winding of transformer39. In the absence of a synchronizing burst, the continuous referencesignal, which is supplied to the phase detector 28 by way of a condenser44, energizes the two detector circuits equally so that the averagepotential at the common point 45 remains at volts. Now, if thesynchronizing burst is of the same frequency and in phase quadrature,that is, 90 out-of-phase with the reference signal, then this burst, asit appears across the two halves of the transformer 39 secondarywinding, combines with the reference signal to produce resultant signalsof equal amplitude across the two detector circuits. The direct-currentpotential point 45 then can remain stable at 0 volts and the oscillator31 is said to be in synchronism with the synchronizing burst.

If the synchronizing burst is of the same frequency but not in phasequadrature, then the resultant signal across one of the detectorcircuits exceeds that across the other. This, in turn, causes adirect-current potential to appear at the common point or terminal 45,the polarity of the potential depending on whether the locally generatedref- 4 erence signal is leading or lagging the desired quadraturerelationship with the synchronizing burst. As the synchronizing burst isnot continuous in nature but rather comes in periodic bursts, the signalat the terminal 45 tends to be periodically applied as pulses. Theserelatively high frequency pulses (15 kc. line pulse rate) are averagedout or smoothed out by the condensers 37 and 41 as well as by the APCfilter 29 which is a form of low-pass filter to develop a correspondingpotential which is then applied to the reactance tube 30 to control theoperating frequency and phase of the oscillator 31 so as to maintain theoutput signal thereof in phase quadrature with the receivedsynchronizing burst. The operation when the synchronizing burst and thelocally generated reference signals are not of the same frequency is abit more complicated and may best be understood by realizing that whentwo signals are not of the same frequency, then they are continuallyvarying in phase relative to one another. As a result, the potential atterminal 45 is continually varying from one extreme to the other. Thispotential variation is commonly referred to as a beat note. Because thephase detector 28 is included in a feedback loop, namely the APC loop,the control action of the loop in trying to pull the two signals intosynchronism causes the beat-note variations to be asymmetrical in naturerelative to the zero voltage axis. This results in a direct-currentcomponent which causes the oscillator 31 to pull into synchronism afterwhich the normal phase'control action just discussed is effective tohold the oscillator 31 in synchronism.

A pair of quadrature-phased reference signals for the synchronousdetectors 22 and 23 is obtained by means of the quadrature transformer33 which includes a primary winding included in a tuned circuit 48 and asecondary winding included in a tuned circuit 49. This circuit 33produces a phase shift at resonance, in this case at 3.6 megacycles. Inother words, the signal across the primary tuned circuit 48 is in phasewith the oscillator whereas the signal across the secondary tunedcircuit 49 is in phase quadrature.

Automatic-phase-control systems or APC loops such as that just describedare described in greater detail in a pair of technical articles writtenby applicant, both appearing in the January, 1954 issue of theProceedings of the I.R.E. One article is entitled Color-CarrierReference Phase Synchronization Accuracy in NTSC Color Television andappears at page 106 while the other article is entitled The DCQuadricorrelator: A Two- Mode Synchronization System and appears at page288. Such systems are also described in chapter 10, pages 170499,inclusive, of a book entitled Principles of Color Television by TheHazeltine Laboratories Staff, published by John Wiley & Sons, Inc., NewYork, 1956. Accordingly, such systems need not be discussed in greaterdetail herein.

Description of ACC system of FIG. 1

Considering now the automatic-chrominance-control system of FIG. 1 whichillustrates a representative embodiment of the present invention, suchsystem includes a chrominance-signal amplifier stage, represented by theband-pass amplifier 20, for translating a received chro minance signalincluding the subcarrier synchronizing bursts. More particularly, suchamplifier stage 20 includes an electron-discharge device such as avacuum tube 50 for amplifying the chrominance signal. Such signal isthen coupled by way of a transformer 51 to a tuned circuit 52 whichincludes the secondary of the transformer 51. Transformer 51 alsoincludes a burst take-01f winding 53. The amplifier 20 also includes aself-biasing network comprising a resistor 54 and a condenser 55 coupledto the cathode of tube 50.

The ACC system of FIG. 1 also includes a second stage for furtherprocessing the chrominance signal. As represented in the FIG. 1embodiment, this second stage may comprise a second chrominance-signalamplifier stage 21 which includes an electron-discharge device or vacuumtube 58 for further amplifying the chrominance signal. The input circuitof amplifier 21 is coupled to the band-pass amplifier 20 by way of thepotentiometer 59 while the output circuit thereof is coupled to thesynchronous detectors 22 and 23 by way of an output transformer 60.

The ACC system may also include a gated amplifier circuit coupled to theoutput of the first amplifier stage 20 for separating and translatingthe subcarrier synchronizing burst portion of the received chrominancesignal. Such gated amplifier circuit is represented by the gated burstamplifier 26.

The ACC system further includes detector circuit means coupled to theoutput of the amplifier stage 20, in this case by way of the gated burstamplifier 26, and responsive to the subcarrier synchronizing bursts fordeveloping a control potential representative of the amplitude of thebursts. As shown in the FIG. 1 embodiment, this detector circuit meansmay take the form of an amplitude-sensitive detector circuit such as theAPC phase detector 28, in which case the voltage at one terminal of theAPC phase detector 28 may be used as the ACC control potential. Asshown, the voltage across the resistor 38 is so used. As an alternative,a separate diode detector, separate and apart from the APC phasedetector 28, might, instead, be used to rectify the sync bursts todevelop the desired control potential. Where possible, however, it ismore economical to make use of acircuit already present in the receiver.

The ACC system also includes circuit means for supplying the controlpotential developed by the last-mentioned detector circuit means to thesecond stage represented, in this case, by the amplifier 21 to disablethis stage when the burst amplitude is insuflicient and to develop avoltage-delayed output potential when the burst amplitude is suflicient.This circuit means includes circuit means for biasing theelectron-discharge device 58 of the second stage 21 to a nonconductivecondition. Such bias circuit means may include a resistor 62 and asource of biasing potential +E A by-pass condenser 63 may also beprovided for by-passing the cathode of tube 58 to ground for thechrominance signal. This circuit means also includes circuit means forsupplying the control signal to the electron-discharge device 58 tocause conduction therein and to control the magnitude thereof. Suchmeans may include a conductor 64 connected between the phase detector 28and the potentiometer 59 as Well as the potentiometer 59 itself.

The ACC system also includes circuit means for supplying thevoltage-delayed output potential of the second stage back to the firstchrominance-signal amplifier stage 20 in a degenerative manner forstabilizing the gain of the chrominance signal. This circuit means mayinclude resistors 65 and 66 which are responsive to the conduction ofthe tube 58 for developing the output potential and may also include aconductor 67 for applying the delayed output or control potential backto the cathode of the tube 50. The polarity of this direct-currentcontrol potential is such that when applied to the cathode of the tube50 it tends to reduce the gain of such tube. In the embodiment shown,this potential is of positive polarity. Where desired, however, thefeedback connections between the amplifiers 21 and 20 may be rearrangedto develop a control potential of negative polarity which may then beapplied to a control electrode such as the control grid or the screengrid of the tube 50.

Operation of ACC system of FIG. 1

Considering now the operation of the automatic-chrominance-controlsystem just described, the composite but time-spaced chrominance signalsand subcarrier synchronizing bursts are supplied by way of terminal 20ato the band-pass amplifier 20. The chrominance signal is then suppliedby way of transformer 51 and tuned circuit 52 to the secondchrominance-signal amplifier 21. The subcarrier synchronizing burst issupplied by way of the burst take-off winding 53 to the gated burstamplifier 26 which is gated on only during the synchronizing burstintervals by the line fiyback pulses supplied thereto by way of terminal26a. In this manner, only the periodic subcarrier synchronizing burstsare supplied to the APC phase detector 28.

Considering the operation of the tion of the phase detector 28 which isused to develop the control potential, that is, the ACC control bias, adirect-current potential or ACC control bias is developed by the upperdetector circuit across the condenser 37 as a result of detection of thesynchronizing bursts. As indicated, this ACC potential is of positivepolarity and its magnitude is related to the amplitude of the subcarrierbursts supplied by the burst amplifier 26. The relationship between theburst amplitude S and the ACC bias B developed across the condenser 37is shown in the graph of FIG. 2. The potential level V represented bythe broken line 70 is the residual direct-current potential developedacross the condenser 37 due to rectification of the locally generatedreference signal supplied to the phase detector 28 by way of thecondenser 44. Curve 71 represents the additional potential resultingfrom the presence of the subcarrier bursts.

The ACC control potential developed across the condenser 37 is thensupplied by way of the conductor 64 to the control electrode of the tube58 of amplifier 21. As mentioned, this amplifier is normally biased to anonconductive condition by means of the bias voltage +E supplied by wayof the resistor 62. This bias level is indicated by the broken line 72of the FIG. 2 graph. As a result, as the amplitude of the subcarrierburst increases, a point is reached at which the tube 58 is renderedconductive. Subsequent increases in the ACC control potential causecorresponding increases in the conduction through the tube 58. Currentflow through tube 58 passes through the resistors 65 and 66 and developsat the junction thereof a direct-current potential which is proportionalto the ACC control potential supplied to the control electrode of thetube 58. Because of the cathode connection, this potential is likewiseof positive polarity. Alternating-current components making up thechrominance signal are by-passed around these resistors by the condenser63. In this manner, there is developed at the junction of the resistors65 and 66 an ACC control potential which is proportional to theamplitude of the subcarrier burst. This ACC control potential is thensupplied back by way of the conductor 67 to the cathode of the tube 50of the band-pass amplifier 20 to stabilize the gain of the chrominancesignal translated thereby. This control potential is of such polarity asto reduce the gain of the tube 50.

If the amplitude of the subcarrier burst supplied to the band-passamplifier 20 increases, then the amplitude of the ACC control potentialwhich is fed back .to this amplifier increases and, hence tends todecrease the gain of the tube 50 to compensate for the increased burstam-. plitude. Because the burst amplitude is also representative of thechrominance-signal amplitude, this, likewise, compensates for theincreased amplitude of the chrominance signal. When the inputchrominance and burst signals decrease in amplitude, the reverse type ofmodification occurs. In this manner, both the chrominancesignal gain andthe burst gain are stabilized. As such time, the operating point on theFIG. 2. graph might be represented by point P The voltage delay affordedby the biasing of the amplifier stage 21 serves to enhance theautomatic-gain-control action by increasing its sensitivity in a mannersimilar to conventional delayed automatic-gain-control systems. In otherwords, when the signal level is below the biasing level 72, a fixedgain-control potential is supplied back to the first amplifier stage 20.As a result, the retarding detector circuit poraction of thegain-control loop in opposing further increases in signal amplitude isnot present. After the signal amplitude increases above the biasinglevel 72, then the automatic control system goes into operation veryrapidly and tends to hold the signal gain constant. This strong andsudden control action represents, in effect, a higher degree of loopgain. An ideal gain-control system would hold the gain constant atapproximately the biasing level 72 but such ideal systems requiresubstantial amounts of loop gain and are not economically feasible inreceivers for the competitive market. Accordingly, as indicated in FIG.2, the system would more likely be stabilized at a point P due to thelack of perfect control action.

The ACC control potential developed across the condenser 37 in additionto serving as an ACC control potential also acts as acolor-killercontrol potential and serves to disable thechrominance-signal channel when other than a color signal is beingreceived. This occurs because at such times no subcarrier synchronizingburst is received and, hence, the ACC control potential is less than thebiasing potential +E As a result, the second amplifier 21 is disabledthus disabling the chrominance-signal channel. This dual function pointsup the primary feature of the present invention, namely, that acolor-killer circuit is normally required in a color receiver and,hence, by combining the ACC therewith in the novel manner taught by thepresent invention, delayed ACC is obtained at no added expense.

Description and operation of ACC system of FIG. 3

Referring now to FIG. 3 of the drawings, there is shown a modified formof chrominance-signal decoder which may be used in the color-televisionreceiver of FIG. 1. The units of the FIG. 3 decoder which are the sameas those of the FIG. 1 decoder are indicated by corresponding referencenumerals. The FIG. 3 decoder may be substituted in the FIG. 1 colorreceiver by connecting the terminals 20a and 24a-24c, inclusive, to thecorrespondingly designated terminals of the FIG. 1 receiver. The FIG. 3decoder includes two major modifications of the decoder of FIG. 1 andeither of these modifications may be made independently of the other.

The first modification relates to the chrominance-channel stage which isutilized to provide the voltage delay for the ACC control potential. Inthe FIG. 3 decoder such second stage for providing the voltage delay isthe chrominance-signal synchronous demodulator stage, which stageincludes a pair of synchronous detectors 22a and 23a corresponding tothe synchronous detectors 22 and 23 of FIG. 1. In this case, the ACCcontrol potential is supplied by way of a conductor 64a and a resistor75 to the control electrodes of each synchronous detector. Thesynchronous detectors 22a and 23a are normally biased to a nonconductivecondition by the biasing potential +E supplied by way of a resistor 76to the cathodes thereof. A by-pass condenser 77 is also coupled to thecathodes as are series-connected resistors 78 and 79 at the junction ofwhich is developed the delayed ACC control potential which is suppliedback to the first stage represented by the band-pass amplifier 20.

With regard to the normal operation of the synchronous detectors, thelocally generated and quadrature-phase reference signals are supplied tothe suppressor electrodes of the synchronous detectors 22a and 23a asindicated in the drawings. The corresponding color-difference signalsare then supplied by way of low-pass filters 80 and 81, respectively, tothe matrix 24.

As illustrated by the foregoing, the voltage delay for the ACC controlpotential may be developed by any one of the various circuits or stagessubsequent to the stage which is being controlled by the ACC controlpotential. Color killing is obtained as before but in this case it isthe synchronous detectors 22a and 23a which are disabled.

Another modification shown in the FIG. 3 decoder relates to the meansutilized for developing the ACC control potential in the first place. Inthis decoder, a portion of the APC phase detector 28 is not utilized asan envelope detector but, rather, a separate synchronous phase detector84 is utilized to develop the ACC control potential. Circuit-wise, thisphase detector 84 may be similar to the phase detector 28 shown indetail in FIG. 1 in that it may include a pair of diode detectorcircuits. One of these circuits includes a diode 85, a condenser 86, aresistor 87, and the upper half of the transformer 88 secondary windingwhile the other diode detector circuit includes a diode 90, a condenser91, a resistor 92, as well as the lower half of the transformer 88secondary winding, There are however, two important distinctions. Onedistinction relates to the phase of the reference signal supplied backto the phase detector 84. More particularly, when the oscillator 31 ofthe APC loop is in synchronism with the received subcarrier burst, thelocal reference signal which is supplied by way of a condenser 94 to thephase detector 84 includes a substantial component which has a phaseidentical to the phase of the synchronizing burst. This results in adifferent mode of operation from that of the APC phase detector 28, itbeing remembered that the reference signal supplied to the APC detector28 is in phase quadrature with the burst. Another distinction of phasedetector 84 over the APC phase detector 28 is the point in the circuitat which the ACC control potential is derived. In the case of the phasedetector 84 the control potential is developed at the terminal 95. Inthis case, the ACC control potential represents the resultant obtainedby combining the individual signals developed by the two diode detectorcircuits of the phase detector 84.

Because of the phase of the locally generated reference signal, theoperating characteristic relative to terminal 95 of the phase detector84 is such that the inphase component produces a maximum output when theAPC loop is in synchronism and substantially zero average output whennot in synchronism. When the APC loop is in synchronism, the magnitudeof the output of phase detector 84 is determined by' the amplitude ofthe input synchronizing burst and the relationship therebetween is shownby curve 99 of the graph of FIG. 4. Such output bias or ACC controlpotential at the terminal 95 is supplied by way of a low-pass filter 96,which may include a resistor 97 and a condenser 98, to the appropriatestage for furnishing the voltage delay and, then, to the amplifier stagewhich is to be controlled. In this manner, the operation of the ACCsystem may be maintained, for example, at the operating point P of FIG.4. The operation of the ACC system during oscillator synchronism issubstantially similar to the operation of the FIG. 1 system. Anyvariation in the amplitude of the input synchronizing bursts at theterminal 2011 results in an inverse variation of the gain of thebandpass amplifier 20. The control action for weak signals is againenhanced by the voltage delay provided by the biasing of the secondstage, in this case the stage comprising the synchronous detectors 22aand 23a. Colorkilling operation, which in this case involves disablingthe synchronous detectors 22a and 23a, is also analogous to that of theFIG. 1 system.

One advantage of the separate phase detector 84 arises when the localoscillator 31 is out of synchronism. In this case, the phase detector 84produces 0 volts of direct-current output on the output side of thelow-pass filter 96 even in the presence of much noise. Actually, duringsuch nonsynchronous condition, the synchronizing burst and the localreference signals supplied to the phase detector 84 produce a sinusoidalbeat-note variation. This variation, however, is symmetrical in waveform, does not produce a direct-current component, and, hence, resultsin no signal component which passes through the low-pass filter 96.

With reference to FIG. 4, the operating point at this time isrepresented by a point P Accordingly, no ACC control bias is suppliedback to the band-pass amplifier 20. This enables the amplifier 20 tohave maximum gain and, hence, to supply subcarrier synchronizing burstsof maximum amplitude to the APC loop as Well as to the additional phasedetector 84. This increase in the amplitude of the burst supplied to theAPC loop renders the operation of such loop more sensitive and, hence,enables the oscillator 31 to be pulled into synchronism more quickly.This increase in sensitivity of the APC loop when the oscillator is notin synchronism represents a two-mode type of operation as discussed morefully in the previously mentioned technical articles and is highlydesirable for such APC loops. In this manner, the ACC system of FIG. 3,which might be termed a synchronous ACC system because it only providesan ACC control potential when the local subcarrier oscillator 31 is insynchronism, also produces, at no additional ex pense, the desirabletwo-mode operation of the APC loop. As soon as the APC loop is pulledinto synchonism, a direct-current output potential appears at terminal95 and the operating point on the FIG. 4 graph immediately shifts topoint P The operating point is subsequently shifted to the point P dueto the ACC control action.

Another advantage of the separate phase detector 84 is that the ACCcontrol action is more immune to undesired electrical noise as suchnoise is averaged out by the circuit.

The present system also improves the color-killer action in that thechrominance-signal channel, in particular the synchronous detectors 22aand 23a, is now disabled when the local oscillator 31 is not insynchronism with the burst. This is because no ACC control bias is thenbeing supplied to overcome the bias of the synchronous detectorcathodes. In this manner, the color killing might also be termedsynchronous color killing as it depends on the state of synchronism ofthe local oscillator 31.

The use of the separate phase detector 84 in developing the ACC controlpotential represents a novel invention apart from whether the furtherfeature of delayed gain-control action is utilized and, hence, has beenmade the subject matter of applicants copending application entitledAutomatic-Chrominance-Control System, Serial No. 622,703, filed November16, 1956, now Patent No. 2,922,839.

FIG. 5 amplifier circuits Referring now to FIG. 5 of the drawings, thereis shown an alternative method of interconnecting the bandpass amplifierand the subsequent amplifier which provides the voltage delay for theACC control potential. The two amplifier circuits 120 and 121 of FIG. 5are similar to the corresponding amplifiers 20 and 21 of FIG. 1 andcorresponding parts are indicated by corresponding reference numeralspreceded, however, by a one. The important feature of theinterconnection of the amplifiers 120 and 121 of FIG. 5 is that thepotential variation at the anode of tube 158 is coupled back by way of aconductor 200 to the screen electrode 201 of the first tube 150. Asbefore, the cathode potential variation of tube 158 may be supplied backby way of a conductor 202 to the cathode of tube 150. The screen bypasscondenser 203 suitably by-passes any chrominancesignal variations sothat the potential fed back to the screen electrode 201 is primarilyrepresentative of the ACC control potential supplied to the terminal204.

The use of the additional feedback path to the screen electrode 201serves to more strongly control the gain of the tube 150. Hence, ineffect, a higher degree of loop gain is provided hence enabling improvedACC control action. The control potential fed back to the screenelectrode 201 by way of conductor 200 is degenerative in nature because,for example, if the burst amplitude increases, then the ACC potentialsupplied by way of terminal 204 increases conduction in tube 158 which,in turn, decreases the anode voltage which, in turn, decreases thepotential of screen electrode 201 hence decreasing the gain of tube and,correspondingly, the amplitude of the burst signal translated by tube150.

Conclusion From the foregoing descriptions of the various embodiments ofthe invention, it will be apparent that an automatic-chrominance-controlsystem constructed in accordance with the present invention represents anovel and attractive way of combining the ACC and color-killer circuitryto obtain delayed ACC operation without increased expense or circuitcomplexity over that required for the normal color-killer circuit.

While there have been described what are at present considered to be thepreferred embodiments of this invention, it will be obvious to thoseskilled in the art that various changes and modifications may be madetherein without departing from the invention, and it is, therefore,aimed to cover all such changes and modifications as fall within thetrue spirit and scope of the invention.

What is claimed is:

An automatic-chrominance-control system for use in thechrominance-signal decoder of a color-television receiver, the systemcomprising: a chrominance-signal amplifier stage for translating areceived chrominance signal including the subscriber synchronizingbursts; a chrominance-signal synchronous demodulator stage for detectinga pair of color-difference signal components of the chrominance signal;detector circuit means coupled to the output of the amplifier stage andresponsive to the subcarrier synchronizing bursts for developing acontrol potential representative of the amplitude of the bursts; circuitmeans for supplying the control potential to the synchronous demodulatorstage to disable this stage when the burst amplitude is insufficient andto develop a voltage-delayed output potential when the burst amplitudeis sufficient; and circuit means for supplying the voltagedelayed outputpotential back to the amplifier stage in a degenerative manner forstabilizing the gain of the chrominance signal.

No references cited.

